Double double balanced mixer

ABSTRACT

A double balanced mixer for mixing an RF input signal with a local oscillator signal to provide at an output an intermediate frequency signal with high isolation over a wide frequency range. The mixer includes a local oscillator balun that is operable to receive a local oscillator signal. A RF balun that is operable to receive a RF signal. A first mixer has a first input port that is coupled to the local oscillator balun and a second input port that is coupled to the RF balun. The first mixer has an output port to provide an intermediate frequency signal. A second mixer is coupled in parallel with the first mixer. The second mixer has a first input port that is coupled to the local oscillator balun and a second input port that is coupled to the RF balun. The second mixer has an output port that provides an intermediate frequency signal. An intermediate frequency balun is coupled to the output ports of the first and second mixers.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to mixers in general and more particularlyto a double balanced mixer that provides increased isolation.

[0003] 2. Description of the Prior Art

[0004] A mixer circuit converts a radio frequency (RF) signal to anintermediate frequency (IF) signal which is the difference of the RF anda local oscillator (LO) signal. The IF frequency is obtained bymultiplying the RF signal with the local oscillator (LO) signal. Thedifference or IF frequency is a result of the non-linearity of themixer. Along with the IF frequency, the mixer typically generatesintermodulation products due to the non-linearity response.

[0005] Isolation is a measure of the circuit balance within the mixer.When the isolation is high, the amount of leakage or feed throughbetween the mixer ports will be very small. Typically, isolationdecreases as frequency increases due to the unbalance in the mixercircuit. Isolation can be measured as LO-RF isolation which is theamount the LO drive level is attenuated when it is measured at the RFport. LO to IF isolation is the amount the LO drive level is attenuatedwhen it is measured at the IF port.

[0006] Mixers are typically designed with one of three topologies:single ended, balanced, and double balanced. The double balanced mixersare capable of isolating both the RF signal and the local oscillator LOvoltages from the output and thus allow overlap of the RF and IFfrequency bandwidths. Several prior art mixer circuits are well known.One mixer design uses a schottky diode quad or ring circuit that usesfour diodes with all of the diodes pointed in the same direction.Another mixer circuit is called a star circuit, which uses two diodespointing toward the central node and two diodes pointing away from thecentral node.

[0007] Double balanced and double-double balanced mixers have beendescribed in the prior art. Diode-type double-balanced mixers belong tothe general classification of “Resistive Switching” mixers. Referring toFIG. 1, a prior art four diode mixer 100 is shown. In this type ofmixer, a local oscillator input signal is applied to input port LO thatis sufficiently large to cause strong conduction of the alternate diodepairs D101 and D102 or D103 and D104, thereby changing them from a lowto a high resistance state during each half of the LO cycle. A virtualground is, therefore, switched or commutated between the radiofrequency/intermediate frequency (RF/IF) transformer T102 windings at arate corresponding to the LO frequency. Since this switching causes a180 degree phase reversal of the RF to IF port transmission during eachhalf of the LO cycle, the mixing process is called bi-phase modulation.For low frequency operation, these devices typically use ferrite coreflux coupled transformers which exhibit leakage inductance and straycapacitance which limits their upper frequency operation and have poorisolation. Using the mixer of FIG. 1, overlapping RF-IF or LO-IFfrequency coverage is very difficult to attain because the IF outputencounters both the RF and LO structures in series for the IF signalpath.

[0008] Turning now to FIG. 2, another prior art mixer is shown. A morecomplex eight diode mixer 200 is shown that produces an overlapping IFrange. Mixer 200 has two diode rings DR202 and DR204 that are coupledtogether. Examination of this mixer structure reveals that the LO isswitching two diode pairs at a time which are in series with the RF-IFsignal path. By tracing out the RF to IF signal connections for eachhalf of the LO input cycle, we see that bi-phase modulation is againbeing performed. The IF port can be seen to be an RF and LO null. Theprinciple advantage of this design is its large RF-IF range overlap, butwith twice as many diodes it requires more LO drive.

[0009] These basic mixer types can be further sub-divided intocategories by the nature of their mixing elements as follows:

[0010] Class I. The most common design consists of a pair (or more) ofthe ferrite-core wideband transformers with four diodes connected in a“ring” configuration. Nominally, these components require about +7 dBmLO drive power.

[0011] Class II, Type 1. This type also uses the ring topology with twoseries-connected diodes in each are. The eight diodes may be similar ordifferent. LO drive levels typically range from +13 to +17 dBm.

[0012] Class II, Type 2. These rely on a ring connection, but feature aprecision resistor in series with a single diode in each arm. Thesefour-diode designs are typically driven at +17 dBm.

[0013] Class III. These are essentially Class II, Type 2 circuits with alarge capacitor connected in parallel with the precision seriesresistor, and they are driven by LO signal in the +20 to +30 dBm range.

[0014] Class IV. Termination Insensitive Mixers. This mixer circuit,called TIM, consists of a transmission line hybrid network driving twosets of diodes. Isolation between each hybrid's opposite ports allowsthe LO to independently control the switching action of alternatelyconducting diode sets.

[0015] Referring now to FIG. 3, another prior art double-double balancedmixer 300 is shown. Mixer 300 has a LO balun B1 connected to a pair ofring diodes DR1 and DR2. The LO balun has an input port LO and a pairtransformers T1 and T2. An RF input port RF is coupled to an RF balunB2. RF balun B2 has a pair of transformers T3 and T4. Ring diode DR1 hasdiodes D1, D2, D3 and D4. Ring diode DR2 has diodes D5, D6, D7 and D8.The IF output port IF is coupled to the junction of diodes D1, D4 andD5, D8. The mixer of FIG. 3, is better suited to low frequency operationand does not have high LO to RF or LO to IF isolation over a widefrequency range.

[0016] While double balanced mixers have been used, they have sufferedfrom not being able to provide high isolation over a wide frequencyrange. A current unmet need exists for an improved double balanced mixerthat has high isolation over a wide frequency range.

SUMMARY OF THE INVENTION

[0017] It is a feature of the invention to provide a double balancedmixer for mixing an RF input signal with a local oscillator signal toprovide at an output an intermediate frequency signal with highisolation over a wide frequency range.

[0018] Another feature of the invention is to provide a double balancedmixer that includes a local oscillator balun that is operable to receivea local oscillator signal. A RF balun that is operable to receive a RFsignal. A first mixer has a first input port that is coupled to thelocal oscillator balun. A second input port is coupled to the RF balunand an output port provides an intermediate frequency signal. A secondmixer is coupled in parallel with the first mixer. The second mixer hasa first input port that is coupled to the local oscillator balun. Asecond input port that is coupled to the RF balun and an output portthat provides an intermediate frequency signal. An intermediatefrequency balun is coupled to the output ports of the first and secondmixers.

[0019] Another feature of the invention is to provide a double-doublebalanced mixer having improved operational characteristics over theprior art.

[0020] A further object of the invention is to provide a double-doublebalanced mixer which uses three pairs of balun transformers along with apair of diode rings to extend the operating frequency range of themixer.

[0021] The invention resides not in any one of these features per se,but rather in the particular combination of all of them herein disclosedand claimed. Those skilled in the art will appreciate that theconception, upon which this disclosure is based, may readily be utilizedas a basis for the designing of other structures, methods and systemsfor carrying out the several purposes of the present invention. Further,the abstract is neither intended to define the invention of theapplication, which is measured by the claims, neither is it intended tobe limiting as to the scope of the invention in any way.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order that the invention may be more fully understood, it willnow be described, by way of example, with reference to the accompanyingdrawings in which:

[0023]FIG. 1 is a schematic drawing of a prior art mixer.

[0024]FIG. 2 is a schematic drawing of another prior art mixer.

[0025]FIG. 3 is a schematic drawing of yet another prior art mixer.

[0026]FIG. 4 is a schematic drawing of the preferred embodiment of thepresent invention.

[0027]FIG. 5 is a schematic drawing illustrating the operation of thepresent invention.

[0028]FIG. 6 is a schematic drawing illustrating the operation of thepresent invention.

[0029]FIG. 7 is a graph showing how the IF output signal responds to theLO signal.

[0030] It is noted that the drawings of the invention are not to scale.In the drawings, like numbering represents like elements between thedrawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0031]FIG. 4 shows a schematic drawing of the preferred embodiment ofthe present invention. Double-double balanced mixer 400 has a localoscillator input terminal LO for receiving a local oscillator signal, anRF input terminal RF for receiving an RF signal and an intermediatefrequency output terminal IF for providing an intermediate frequencyoutput signal.

[0032] A first mixer or diode ring DR1 has diodes D1, D2, D3 and D4.Each diode has an anode and a cathode. The cathode of diode D1 isconnected to the anode of diode D2 at a port or node N1. The cathode ofdiode D2 is connected to the anode of diode D3 at a port or node N2. Thecathode of diode D3 is connected to the anode of diode D4 at a port ornode N3. The cathode of diode D4 is connected to the anode of diode D1at a port or node N4.

[0033] A second mixer or diode ring DR2 has diodes D5, D6, D7 and D8.Each diode has an anode and a cathode. The cathode of diode D5 isconnected to the anode of diode D6 at a node N5. The cathode of diode D6is connected to the anode of diode D7 at a node N6. The cathode of diodeD7 is connected to the anode of diode D8 at a node N7. The cathode ofdiode D8 is connected to the anode of diode D5 at a node N8. Mixers DR1and DR2 are effectively coupled in parallel.

[0034] Local oscillator port LO is connected to local oscillator balunB1. Balun B1 has a pair of transformers T1 and T2. Transformer T1 haswire windings W1 and W2. Transformer T2 has wire windings W3 and W4.Windings W1-W4 are wound on a figure eight shaped core (not shown) thatare commonly used with baluns. Winding W1 has one end connected to portLO and the other end connected to nodes N1 and N5. Winding W2 has bothends connected to ground. Winding W3 has one end connected to ground andthe other end connected to port LO. Winding W4 has one end connected tonode N3 and N7 and the other end connected to ground. The dots on thewindings identify its polarity with respect to another winding. Thus thelocal oscillator signal is connected to the mixers through baluns B1.

[0035] Intermediate frequency output port IF is connected to IF balunB3. Balun B3 has a pair of transformers T5 and T6. Balun B3 has wirewindings W9, W10, W11 and W12. Windings W9-W12 are wound on a figureeight shaped core (not shown) that are commonly used with baluns.Winding W9 has one end connected to node N4. Winding W10 has one endconnected to the IF port IF and the other end connected to node N8.Winding W11 has one end connected to IF port IF and the other endconnected to node N2. Winding W12 has one end connected to node N6. Thusthe Intermediate frequency signal IF is provided to output port IFthrough balun B3.

[0036] RF input port RF is connected to RF balun B2. Balun B2 has a pairof transformers T3 and T4. Balun B2 has wire windings W5, W6, W7 and W8.Windings W5-W8 are wound on a figure eight shaped core (not shown) thatare commonly used with baluns. Winding W5 has one end connected to portRF and the other end connected to winding W9. Winding W6 has both endsconnected to ground. Winding W7 has one end connected to ground and theother end connected to port RF. Winding W8 has one end connected towinding W12 and the other end connected to ground. Thus, the RF signalis connected to the mixers through balun B2.

[0037] The following examples 1 and 2 are provided to further teach andexplain the present invention.

EXAMPLE 1

[0038] Referring to FIGS. 4 and 5, the operation of the circuit of FIG.4 when the local oscillator signal is positive relative to ground isshown. Assume that all of the diodes are symmetrical. During thepositive half of the LO signal, diodes, D2, D3, D6 and D7 conduct whilediodes D1, D4, D5 and D8 are at cutoff. In this case, nodes N2 and N6are virtually grounded while nodes N4 and N8 are disconnected. Theequivalent circuit is shown in FIG. 5. The negative −RF signal istransformed to the IF signal. The IF output is equal to the negative −RFsignal.

EXAMPLE 2

[0039] Referring to FIGS. 4 and 6, the operation of the circuit of FIG.4 when the local oscillator signal is negative relative to ground isshown. Assume that all of the diodes are symmetrical. During thenegative half of the LO signal, diodes, D1, D4, D5 and D8 conduct whilediodes D2, D3, D6 and D7 are at cutoff. In this case, nodes N4 and N8are virtually grounded while nodes N2 and N6 are disconnected. Theequivalent circuit is shown in FIG. 6. The positive +RF signal istransformed to the IF signal. The IF output is equal to the positive +RFsignal.

[0040] Referring to FIG. 7, a graph showing how the IF output signalresponds to the LO signal is shown. When the LO signal is positive, theIF signal is equal to the negative RF signal. When the LO signal isnegative, the IF signal is equal to the positive RF signal.

[0041] The present invention has several advantages. Since, nodes N4 andN8 or N2 and N6 are virtually grounded during operation, only remnantsor leakage signals due to parasitic elements of the local oscillatorsignal LO arrive to these nodes. The leakage local oscillator LO signalsarriving at these nodes have the same amplitude and phase. When thesesignals arrive at nodes N4 and N8 or N2 and N6, they are thereforecancelled at the RF and IF ports because of the baluns B2 and B3. Thepresent invention provides “double isolation” of the local oscillatorsignal LO from the RF and IF ports. The configuration of these baluns isself compensating. The mixer of FIG. 4 provides a high LO to RF and LOto IF isolation over a wide frequency range.

[0042] While the invention has been taught with specific reference tothese embodiments, someone skilled in the art will recognize thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. The described embodiments are to beconsidered in all respects only as illustrative and not restrictive. Thescope of the invention is, therefore, indicated by the appended claimsrather than by the description. All changes that come within the meaningand range of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A double balanced mixer for mixing an RF inputsignal with a local oscillator signal to provide at an output anintermediate frequency signal, comprising: (a) a first diode ring havinga first and second input port for receiving the local oscillator signaland a third input port for receiving the RF signal, the first diode ringfurther having a first output port for providing the intermediatefrequency signal; (b) a second diode ring having a fifth and sixth inputport for receiving the local oscillator signal and a seventh input portfor receiving the RF signal, the second diode ring further having athird output port for providing the intermediate frequency signal; (c) afirst balun connected across the first and second input ports and thefifth and sixth input ports for receiving the local oscillator signal;(d) a second balun connected to the third and seventh input ports forreceiving the RF signal; and (e) a third balun connected to the firstand third output ports for providing the intermediate frequency signal.2. The double balanced mixer according to claim 1, wherein the firstbalun includes a first transformer that has a first and a second windingand a second transformer that has a third and fourth winding.
 3. Thedouble balanced mixer according to claim 1, wherein the second balunincludes a third transformer that has a fifth and a sixth winding and afourth transformer that has a seventh and an eighth winding.
 4. Thedouble balanced mixer according to claim 1, wherein the third balunincludes a fifth transformer that has a ninth and a tenth winding and asixth transformer that has an eleventh and a twelfth winding.
 5. Thedouble balanced mixer according to claim 1, wherein each diode ringcomprises: a) a first diode having an anode and a cathode; b) a seconddiode having an anode and a cathode, the cathode of the first diodeconnected to the anode of the second diode; c) a third diode having ananode and a cathode, the cathode of the second diode connected to theanode of the third diode; and d) a fourth diode having an anode and acathode, the cathode of the third diode connected to the anode of thefourth diode and the cathode of the fourth diode connected to the anodeof the first diode.
 6. The double balanced mixer according to claim 1,wherein parasitic elements of the local oscillator signal are cancelledin the second and third baluns.
 7. The double balanced mixer accordingto claim 1, wherein isolation between the local oscillator signal andthe RF and intermediate frequency signals is increased.
 8. A doublebalanced mixer, comprising: (a) a local oscillator balun operable toreceive a local oscillator signal; (b) a RF balun operable to receive aRF signal; (c) a first mixer having a first input port coupled to thelocal oscillator balun, a second input port coupled to the RF balun, andan output port providing an intermediate frequency signal; (d) a secondmixer coupled in parallel with the first mixer, the second mixer havinga first input port coupled to the local oscillator balun, a second inputport coupled to the RF balun, and an output port providing anintermediate frequency signal; and (e) an intermediate frequency baluncoupled to the output ports of the first and second mixers.
 9. Thedouble balanced mixer according to claim 8, wherein the first and secondmixers are each ring diodes.
 10. The double balanced mixer according toclaim 9, wherein the ring diodes each comprise four diodes.
 11. Thedouble balanced mixer according to claim 10, wherein the baluns eachhave a pair of transformers.
 12. The double balanced mixer according toclaim 11, wherein the transformers each have a pair of windings.
 13. Thedouble balanced mixer according to claim 8, wherein parasitic elementsof the local oscillator signal are cancelled in the RF and intermediatefrequency baluns.
 14. The double balanced mixer according to claim 8,wherein isolation between the local oscillator signal and the RF andintermediate frequency signals is increased.
 15. A method of doublebalanced mixing, comprising: (a) applying a local oscillator signalthrough a local oscillator balun to a first and second double balancedmixer, the first and second mixers coupled to each other; (b) applyingan RF signal through an RF balun to the first and second mixers; (c)mixing the local oscillator signal with the RF signal in the first andsecond mixers to obtain an intermediate frequency signal; and (d)providing the intermediate frequency signal through an intermediatefrequency balun to an output port.
 16. The method of double balancedmixing according to claim 15, wherein the first and second mixers arecoupled in parallel.
 17. The method of double balanced mixing accordingto claim 16, wherein the first and second mixers each are four diodescoupled in a ring configuration.
 18. A double balanced mixer for mixingan RF signal with a local oscillator signal to provide at an output anintermediate frequency signal, comprising: (a) local oscillator balunmeans for receiving a local oscillator signal; (b) RF balun means forreceiving an RF signal; (c) mixer means, the mixer means coupled to thelocal oscillator balun means and the RF balun means, respectively, themixer means mixing the local oscillator signal and the RF signal toproduce an intermediate frequency signal; and (d) intermediate frequencybalun means coupled to the mixer means.
 19. The double balanced mixeraccording to claim 18, wherein the mixer means is a first and secondmixer connected in parallel.
 20. The double balanced mixer according toclaim 19, wherein the first and second mixers each are four diodescoupled in a ring configuration.
 21. The double balanced mixer accordingto claim 20, wherein parasitic elements of the local oscillator signalare canceled in the RF and intermediate frequency balun means such thatisolation between the local oscillator signal and the RF andintermediate frequency signals is increased.